Trough waveguide slow wave antennas and transmission lines



w. ROTMAN 3,013,267 TROUGH WAVEGUIDE SLOW WAVE ANTENNAS AND TRANSMISSION LINES Dec. 12,1961

Filed March 20, 1957 QwQQvQ IN VENTOR. M44729? Raf/YA United States Patent ()fiice 3,013,267 Patented Dec. 12, 1961 3,613,267 TRQUGH WAVEGUIDE SLOW WAVE ANTENNAS AND TRANdMISSIGN LINES Walter Rotman, 3 (Ihiswick Road, Brighton, Mass. Fiied Mar. 20, 1957, Ser. No. 647,455 3 Claims. (Cl. 343-772) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates generally to antennas and transmission lines and more particularly to apparatus for the transmission of high-frequency electrical energy. The invention is characterized by the utilization of a special wave-guide structure as a means for transmitting wave energy and radiating energy to the surrounding space and involves the use of elements Within the waveguide structure for controlling the guide Wavelength or phase velocity and/or radiation intensity or attenuation rate in the waveguide. Control of these parameters in effect enables the obtaining of any desired controlled radiation pattern.

A trough waveguide of either rectangular channel shape, U-shape, V-shape or variations thereof having a symmetrically disposed fin therein comprises the trans mission line which is modified to produce the controlled radiation. A basic concept on which a part of this invention relies is that anti-symmetrical obstacles in a trough waveguide couple energy from the bound symmetrical trough guide mode, a transverse electric (TE) mode, into energy in a transverse electromagnetic (TEM) field which radiates into free space from the open side of the waveguide. Discrete symmetrical obstacles, on the other hand, react as tunning elements. In accordance with above principle, a large variety of radiating devices may be constructed.

The trough waveguides, together with their radiating elements, have utility alone or as primary line source arrays for use with secondary reflectors, horns, or lenses in radar and communication antenna systems. In this respect, they are superior to slots or dipole equivalent arrays on rectangular or circular waveguides. When not used for wave propagation into free space the devices have utility as delay lines and frequency dispersive lines for obtaining a variable phase shift for changes in frequency.

Because of the superior electrical and mechanical properties of the antennas of this invention, many new advantages are realized.

It is contemplated that the subject invention will open new vistas in the fields of radio astronomy, radar, and communications.

An object of this invention is to produce a trough waveguide antenna with means for controlling the radiation therefrom.

Another object of this invention involves the production of a novel slow wave transmission line.

Still another object of the inventionis to produce a non-resonant traveling wave array composed of either resonant or non-resonant radiating elements.

A further object of the invention is to produce a novel trough waveguide transmission line suitable for use as a delay line.

A still further object of the invention involves the production of a novel frequency dispersive transmission line.

It is another object of the invention to produce a novel frequency scannable antenna.

It is still another object of the invention to produce a novel traveling Wave leaky waveguide antenna.

It is a further object of this invention to producea novel resonant array of resonant elements.

It is a still further object of the invention to produce a novel trough waveguide antenna array which is polarized in the direction of the center fin of the waveguide.

Another object of the invention involves the production of a traveling wave antenna array of non-resonant radiating elements in which there is achieved a phase reversal .between successive elements.

A further object of the invention involves the production of a novel end fire or near end fire radiation antenna.

A still further object of the invention involves the production of an antenna and/ or transmission line which is easily and economically produced by conventional cornmerical manufacturing techniques.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings, where- FIGURE 1 is a cross-sectional view of a trough waveguide with a representation of the electric field of the transverse electric (TE) mode therein;

FIGURE 2 is a cross-section of a trough waveguide with a representation of the electric components of the transverse electromagnetic (TEM) field;

FIGURE 3 is a pictorial view of a slow wave trough waveguide structure with means in dotted lines for controlling radiation characteristics;

FIGURE 4 is a pictorial view of an alternative embodiment of a slow wave trough waveguide structure with means in dotted lines for controlling radiation characteristics; and

FIGURE 5 is a pictorial view of a vertically polarized traveling wave antenna.

The invention utilizes a symmetrical non-radiating trough waveguide which acts as a transmission line. This type of waveguide in its mechanical construction is left open on one side; however, it still acts as a transmission line. Its characteristics, frequency-wise follow those of a waveguide while it retains mechanical simplicity of a strip transmission line. The general configuration of the waveguide has two side walls and a bottom, thus forming a trough, and a substantially centrally disposed fin member of less height than the side walls. The embodiment of FIGURE 1 is a channel or rectangular form of trough waveguide but various alternative forms can be achieved by deformation of the side walls, for example, the trough may be of U or V shape as long as symmetry about the central fin is maintained in order to avoid spurious modes.

Like numerals will designate like parts of the waveguide throughout the specification.

FIGURE 1, for illustration purposes, shows a rectangular trough waveguide having side walls 20 and 21, a bottom wall 22 and a substantially centrally disposed fin member 23. When the side walls 20 and 21 are less than one half wavelength apart, A TE (transverse electric) mode may be propagated along the axis of the guide. This mode is bound to the center fin and has an electric field with a general configuration as shown in FIG- URE 1. The intensity of the field lines of the electric vector increases from the bottom 22 of the waveguide to the top of the central vane or fin 23. The transverse currents on the sides of fin 23 vary from a minimum at the free edge to a maximum at its base.

The electric components of the TEM (transverse electromagnetic) field is depicted in FIGURE 2 and can be propagated wherever side walls exist. As is shown by the dashed lines at the open end of the trough, radiation into space is achieved whenever the field strikes the open top or aperture of the guide. The creation of an asymmetry causes energy from the TE field to be converted into a TEM field which is not bound to the center fin. The TEM field is capable of being propagated in all directions at right angles to the TE vectors which allows a release of energy from the open side of the Waveguide.

The characteristics of a trough waveguide are such that the cut-off wavelength depends upon the electrical height of the center fin 23, i.e., the cut-off wavelength is approximately that at which the center fin is a quarter wavelength. The side wall (20, 21) height above the center fin 23 and the spacing between side walls act to prevent unwanted uncontrolled radiation. Less than half wavelengths spacing between side walls allows for operation of the line over a range of frequencies. The spacing of side walls, at any rate, should not exceed a half wavelength of the highest frequency in the range. The TE mode is critically dependent upon the dimensions of the center fin while the TEM or radiating mode is independent of the fin 23.

Although the trough waveguide is shown as a unitary structure of highly conductive material, each of portions designated 20, 21, 22 and 23, could be made of separate pieces of stock of any material secured together by conventional means as long as the interior of the trough is plated or otherwise lined with a highly conductive material.

The solid line structure of FIGURE 3 is a representation of a slow wave transmission line suitable for use as a delay line, a frequency dispersive line or a frequency scannable antenna. Whereas discrete vertical posts above the fin 23 act as capacitive shunts when the post height is less than a quarter wavelength above the fin and as inductive shunts when greater than a quarter wavelength above the fin 23; the vertical posts 24, when spaced less than a quarter wavelength apart and of less than a quarter wavelength in height above the fin 23 as shown in FIGURE 3, can be considered to load the trough waveguide periodically with capacitive shunt susceptances. This reactive loading can decrease the phase velocity of the traveling wave to a value less than that of the wave in free space, which results in a slow wave structure. At half wave spacings of the elements a high impedance mismatch results; therefore, to avoid an impedance discontinuity the spacing should be a quarter wavelength or less.

When the ratio of the height of the post 24 to the height of the fin 23 is greater than unity, the transmission line becomes very frequency dispersive, i.e., changes in frequency cause changes in phase velocity.

The embodiment of FIGURE 4 represents the condition wherein the greatest frequency dispersion occurs, i.e., when the fin approaches zero thus leaving only posts 24 in the waveguide. The frequency range of energy transmission of the embodiment of FIGURE 4 will be small and centered approximately at those frequencies for which the post is a quarter wavelength in height. Intermediate fin heights between FIGURES 3 and 4 result in varying degrees of frequency bandpass characteristics. The principle of variation of phase velocity as a function of frequency may be adapted to structures to produce frequency scannable antennas.

In order to utilize the waveguide for wave radiation into free space, it is necessary to introduce means for creating an asymmetry in the cross section of the device as is taught in my co-pending application, Serial No. 647,454 titled: Trough Waveguide Antennas and filed on even date herewith.

The dotted line portions of FIGURES 3 and 4 show examples for creating an asymmetry. In FIGURE 3 a discrete radiating horizontal rod 25 is placed on side wall 2% and a post extension 26 provides for tuning to eliminate reflections. A near end fire radiating antenna array, either resonant or non-resonant, depending on its termination, may 'be constructed by placing the elements 25 and 26 at half wavelength intervals. To obtain broadside or near broadside radiation, phase reversal at half wavelength intervals must be introduced by placing the radiating rods 25 in a staggered relationship on opposite walls 20 and 21.

FIGURE 4 represents an end fire leaky waveguide non-resonant array having a block 27 running the length of the waveguide.

Near broadside or broadside radiation, if desired, may be achieved by alternating blocks on either side of posts 24 for half wavelength intervals. Symmetrical tuning posts may 'be utilized in this type of structure as in FIG- URE 3 to cancel reflections. Although two types of means for radiation are shown, any means conforming to the teachings of my aforementioned co-pending application may be used. A frequency dispersive transmission line with radiating elements becomes a frequency scannable antenna suitable for varying the angle of the radiated beam with changes in input frequency.

The vertically polarized end fire radiating antenna of FIGURE 5, shown with a ground plane 29 unlike the previous embodiments, does not utilize asymmetric means for producing radiation into free space. A large number of vertical posts 28 are placed on fin 23 in a trough waveguide and the post height adjusted such that the velocity of the wave in the guide equals the free space velocity of the wave. Side Walls 20 and 21 are reduced from their normal height until the individual posts 28 protrude above the plane in which the tops of the side walls 20, 21 lie. The amount of projection of posts 28 above this plane determines the amount of radiation into free space that will occur. That part of posts 28 which projects above the side walls acts as monopoles to produce an end fire radiation antenna with an electric polarization in a direction parallel to the fin 23 or monopoles 28.

If the ratio of wavelength in free space to the guide wavelength were varied by adjusting the post height to fin height ratio, near end fire radiation of up to approximately 40 could be realized.

The antenna of FIGURE 5 may also be made frequency scannable between end fire and approximately 40 from end fire when the ratio of post height to fin height is greater than unity.

In order to create a proper impedance match between an input or termination and the antenna or transmission line, the fin height should be made to gradually increase to the post height as it approaches the input or termination line.

Although the invention has been described with reference to particular embodiments, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

1. A slow wave trough waveguide transmission line suitable for use as a delay line comprising a three-sided trough-shaped member, a fin symmetrically disposed in said trough-shaped member and running the length thereof, and a series of posts on said fin and in line therewith, said posts being less than a quarter Wavelength apart and extending less than a quarter Wavelength above said fin.

2. A frequency dispersive transmission line comprising a three-sided trough-shaped member, a centrally disposed fin extending longitudinally thereof, and a series of posts on said fin, the ratio of post height to fin height being greater than unity.

3. A frequency dispersive transmission line as defined in claim 2 including means asymmetric with respect to a lateral cross section of said member for producing radiation from said transmission line, the beam angle of which is varied by changes in frequency.

References Cited in the file of this patent UNITED STATES PATENTS Ratlifi July 8, 1952 6 2,659,817 Cutler Nov. 17, 1953 2,735,958 Brown Feb. 21, 1956 2,799,831 Fubini July 16, 1957 2,806,210 Edwards Sept. 10, 1957 2,829,343 Kostrize et a1 Apr 1, 195 8 2,849,711 MacKimmie Aug. 26, 1958 2,903,656 Weisbaum Sept. 8, 1959 2,943,325 Rotman June 28, 1960 OTHER REFERENCES Pub. I, Some New Microwave Antenna Design Based On The Trough Wave Guide, Rotman and Karas, 1956 IRE Convention Record, vol. 4, part 1, copyright 1956, pgs. 230235.

Pub. II, The Bell System Technical Journal, vol. 34, Number 1, January 1955, pgs. 71 and 72. 

